The etiologic agent of pneumonic plague is the facultative intracellular bacterium Yersinia pestis. As antibiotic-resistant strains of Y. pestis are known to exist and sophisticated bioterrorist attacks are plausible, there is an urgent need to develop pneumonic plague vaccines. Subunit vaccines comprised of the Y. pestis F1 and/or V proteins provide experimental animals with significant protection against pneumonic plague. Further development of these vaccines will undoubtedly be aided by a better understanding of how vaccination protects against plague. While it is clear that antibodies contribute to the protective response, CD4 + T cells must also participate, as antigen-specific CD4 + T cells are critically important for memory B cell responses and the affinity maturation of antibodies. Relevant prior studies have been limited in scope, although they established that the F1 and V proteins do elicit significant CD4 + cell responses. In addition to stimulating, maintaining and/or boosting antibody responses, vaccineelicited CD4 + T cells could also direct cellular immunity at host cells harboring E pestis organisms, and/or pathologically contribute to vaccine-related adverse reactions. Thus, detailed studies of Y. pestis vaccine elicited CD4 + T cells are clearly warranted. Prior to defining the functional capacities of vaccine-elicited CD4 + T cells, one must first develop appropriate model systems. In Aim 1, we will determine the immunodominant epitopes recognized by V protein-specific CD4 + T cells and develop assays for enumerating V-specific T cells. In Aim 2, we will use those assays to define vaccination conditions that optimally prime effector and memory V-specific CD4 + T cells. We will also define conditions that recruit V-specific CD4 + T cells to pulmonary tissues, which are the primary target of pneumonic plague. In Aim 3, we will determine whether the same vaccination conditions optimally prime both B and T cell responses. Those studies may immediately suggest means to improve vaccine efficacy and/or limit adverse reactions. The primary goals of this R21 proposal are to develop assays and vaccination protocols that will enable future studies to measure their capacity of E pestis-specific CD4 + T cells to protect against pneumonic plague, and to dissect the relative importance of vaccine-elicited B and T cell responses. Our long-term goals are to use that information to develop effective pneuinonic plague vaccines.